To produce oil or gas, a well is drilled into a subterranean formation that is an oil or gas reservoir.
Drilling is the process of drilling the wellbore of a well. After a portion of the wellbore is drilled, sections of steel pipe, referred to as casing, which are slightly smaller in diameter than the borehole, are placed and cemented in at least the uppermost portions of the wellbore. After drilling, the casing provides structural integrity to the newly drilled borehole. The wellbore is then completed for production of oil or gas.
The well is created by drilling a hole into the earth (or seabed) with a drilling rig that rotates a drill string with a drilling bit attached to the downward end. Usually the borehole is anywhere between about 5 inches (13 cm) to about 36 inches (91 cm) in diameter. As upper portions are cased or lined, progressively smaller drilling strings and bits must be used to pass through the uphole casings or liners, which steps the borehole down to progressively smaller diameters.
While drilling an oil or gas well, a drilling fluid is circulated downhole through a drillpipe to a drill bit at the downhole end, out through the drill bit into the wellbore, and then back uphole to the surface through the annular path between the tubular drillpipe and the borehole. The purpose of the drilling fluid is to lubricate the drill string, maintain hydrostatic pressure in the wellbore, and carry rock cuttings out from the wellbore.
The drilling fluid can be water-based or oil-based. Oil-based fluids tend to have better lubricating properties than water-based fluids, nevertheless, other factors can mitigate in favor of using a water-based drilling fluid.
An example a water-based drilling fluid is a drilling mud, which can include an aqueous solution and undissolved solids (as solid suspensions). A water-based drilling mud can be based on a brine. The dissolved solids and the undissolved solids can be chosen to help increase the density of the drilling fluid. An example of an undissolved weighting agent is barite (barium sulfate). The density of a drilling mud can be much higher than that of typical seawater or even higher than high-density brines due to the presence of suspended solids.
In drilling, subterranean formations including clay can be encountered. Certain clays, such as montmorillonite, have the tendency to swell when exposed to water, creating a potential drilling hazard when clay-bearing rock formations are exposed to water-based fluids during drilling. Such clay is unstable and exposure to water in a drilling fluid can pose problems such as hydration, solvation, and dispersion of the clay. This instability of the clay can cause erosion and destroy the rock formation. The eroding of swelled clay particulate complicates downhole drilling fluid behavior. In addition, the swelling of clay particles in the formation can reduce or plug the permeability of a reservoir rock.